Optimizing energy transmission in a leadless tissue stimulation system

ABSTRACT

Method and systems for optimizing acoustic energy transmission in implantable devices are disclosed. Transducer elements transmit acoustic locator signals towards a receiver assembly, and the receiver responds with a location signal. The location signal can reveal information related to the location of the receiver and the efficiency of the transmitted acoustic beam received by the receiver. This information enables the transmitter to target the receiver and optimize the acoustic energy transfer between the transmitter and the receiver. The energy can be used for therapeutic purposes, for example, stimulating tissue or for diagnostic purposes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent Application Ser. No.16/107,626, filed Aug. 21, 2018, now U.S. patent No. 10,456,588, whichis a divisional of U.S. patent application Ser. No. 14/221,040, filedMar. 20, 2014, now U.S. patent No. 10,080,903, which is a continuationof U.S. patent application Ser. No. 11/752,775, filed May 23, 2007, theentire content of each of which is incorporated herein by reference inits entirety.

The subject matter of this application is related to that of thefollowing commonly owned patent applications: Ser. No. 11/315,524, Ser.No. 11/535,857, Ser. No. 11/315,023. The full disclosures of each ofthese prior filings are incorporated herein by reference but the benefitof the filing dates is not being claimed.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention generally relates to optimizing acoustic or ultrasoundenergy transmission and energy conversion and more particularly tooptimizing acoustic energy transmission and conversion in implantabledevices.

Stimulation of cardiac tissue using a leadless cardiac stimulationsystem has been disclosed earlier by the applicant. Generally, such asystem comprises an arrangement of one or more acoustic transducers, andassociated circuitry, referred to as a controller-transmitter, and oneor more implanted receiver-stimulator devices. Thecontroller-transmitter generates and transmits acoustic energy, which isreceived by the receiver-stimulator, and the receiver-stimulator in turnconverts the acoustic energy into electrical energy, which is deliveredto the tissue through electrodes.

The controller-transmitter may be externally coupled to the patient'sskin, but will usually be implanted, requiring that thecontroller-transmitter have a reasonable size, similar to that ofimplantable pacemakers, and that the controller-transmitter be capableof operating for a lengthy period, typically three or more years, usingbatteries. The small size and long operational period require that thesystem efficiently utilize the acoustic energy from thecontroller-transmitter with minimal dissipation or dispersion of thetransmitted energy and efficient conversion of the energy by thereceiver-stimulator.

Charych (U.S. Pat. No. 6,798,716) describes various strategies forlocating an acoustic receiver. Charych describes methods for chargingwireless devices (receivers) from a controller-transmitter that ispowered through a plug, providing power in excess of 1000 W. Incontrast, a leadless cardiac stimulation system, where the power flow is6 orders of magnitude lower, requires completely different methods andsystems for locating the receiver, which are not described by Charych.

Briefly, in its simplest form, the receiver-stimulator comprises one ormore acoustic piezoelectric receiver elements, one or more rectifiercircuits, and electrodes. The piezoelectric receiver elements couplepower from the acoustic field generated by the controller-transmitterand convert it into electric power. If applied directly to the tissuethis AC electrical power does not stimulate the tissue because itsfrequency is too high for excitation/stimulation. In order to initiate apaced heart beat, or provide other therapeutic stimulation to tissues,the rectifier circuits convert all or some of the available ACelectrical power to an electrical pulse that is applied to the cardiactissue through the electrodes. The acoustic field is generated andtransmitted either by an externally placed or an implantablecontroller-transmitter that is remote from the location of thereceiver-stimulator.

The acoustic energy generated by the controller-transmitter is generallyreferred to as an acoustic beam or ultrasound beam and is characterizedby acoustic intensity (I) measured in Watts/square meter. In order tocreate an acoustic intensity of Io over an area Ao thecontroller-transmitter must expend at least Io*Ao Watts of power. Onlythe portion of this acoustic beam that intersects thereceiver-stimulator will be available as electrical power. If the areaAo is larger than the cross sectional area or aperture of the receiverAr, then the ratio Ar/Ao represents that fraction of the power in theacoustic beam that is available to the receiver-stimulator. Thereforethe optimally efficient acoustic beam is very narrow and only intersectsthe receiver elements of the receiver-stimulator.

The controller-transmitter has one or more piezoelectric transducersthat convert electrical power into acoustic power creating the acousticbeam that is directed at the receiver-stimulator. The ability of thecontroller-transmitter to generate this acoustic beam over a small areais characterized by its focal or directivity gain. In general the largerthe cross sectional area (referred to as the aperture) of thecontroller-transmitter transducers, the higher the directivity gain willbe. This requires the controller-transmitter to have a wide aperturetransmitter that focuses acoustic energy at the receiver-stimulator. Italso requires the controller-transmitter to steer or direct the acousticbeam at the receiver-stimulator. This can be accomplished by using aphased array that uses beam-forming techniques to steer the acousticbeam at the receiver-stimulator. Steering can be accomplished byadjusting the phases and amplitudes of the electrical drive signals tothe transducer array, which results in adjusting the direction and focaldistance of the transmitted beam.

If the location of the receiver-stimulator or the controller-transmitterdoes not change over time, the controller-transmitter could beconfigured at the time of implant to optimally select a focused beamprofile that is aimed at the receiver-stimulator location determined atthe implantation time. However, in the case of the leadless system, thereceiver-stimulator can be expected to move due to cardiac motion,breathing, or body orientation. Moreover, the controller-transmitter maymove slightly due to body orientation or body movements or migration.Therefore, to accommodate the movement of the controller-transmitter andthe receiver-stimulator, inventors herein have realized that successfuloperation in the simplest implementation would require a relativelybroad beam acoustic emission. However, in this mode of operation most ofthe transmitted acoustic energy may pass by the receiver-stimulator andnot used efficiently. Hence, inventors herein have further realized thatto improve efficiency the transmit beam needs to be significantlysharpened or focused, and reliable operation would require continuous,specific knowledge of the location of the receiver-stimulator.

For the above reasons, it would be desirable to provide a leadlesssystem that efficiently transmits and receives acoustic energy. It wouldalso be desirable for the transmitted beam to be adjusted, to be asfocused as possible at targeting the receiving element(s) of thereceiver-stimulator. It would be particularly desirable if the locationof the receiver-stimulator is known to the controller-transmitter, and,thereby, a focused acoustic beam could be aimed and transmitted towardthe receiver-stimulator. It would also be desirable if thereceiver-stimulator is located using mechanisms that minimize the sizeand complexity of the receiver-stimulator such that additional circuitryor energy consumption is not imposed upon the receiver-stimulator.

SUMMARY OF THE INVENTION

Systems and methods are provided for efficiently delivering acousticenergy from an implanted or externally applied acoustic transmitter toan implanted acoustic receiver. The acoustic energy is converted by thereceiver into electrical energy which can be used for a variety ofpurposes. The electrical energy will typically be delivered toelectrodes in contact with tissue in order to stimulate tissue, forexample, in cardiac pacing for bradycardia, for termination oftachyarrhythmia, for bi-ventricular resynchronization therapy for heartfailure, or the like. The systems and methods of the present inventioncould also be used in a variety of other applications, includingapplications for nerve stimulation, brain stimulation, voluntary musclestimulation, gastric stimulation, bone growth stimulation, painamelioration, sensing and communication of local diagnostic information,and the like, where an acoustic transmitter has to efficiently transmitenergy to an implanted receiver. The implanted acoustic receiver couldact as a tissue stimulator (receiver-stimulator) or act more generallyas an acoustic energy converter (receiver-converter). Efficienttransmission can be achieved by deploying strategies for locating thereceiver and then transmitting a focused acoustic beam specificallyaimed at the receiver and thereby improving operational efficiency ofthe system. These systems and methods are particularly useful when thetransmitter is an implantable device dependent on a limited source ofenergy, such as a battery.

By “locator signal” we mean an acoustic signal transmitted by thetransducer element(s) of a controller-transmitter assembly to elicit a“location signal.”

By “location signal” we mean a signal that is either passively oractively generated by the receiver-stimulator. The location signal maybe in response to a “locator signal” transmitted by thecontroller-transmitter or may be periodically transmitted by thereceiver-stimulator. The location signal is used by thecontroller-transmitter to determine the location of the receiverrelative to the controller-transmitter, thus allowing thecontroller-transmitter to direct a focused, efficient acoustic beam atthe receiver-stimulator.

One exemplary embodiment of the invention is a system for focusingacoustic energy into a human body. The system comprises an array ofacoustic transducers configured to transmit acoustic energy into thebody; circuitry for focusing the acoustic energy at specific regions inthe body; an acoustic receiver adapted to receive the acoustic energyand convert the acoustic energy into electrical energy; a pair ofelectrodes connected to the acoustic receiver and adapted to transferthe electrical energy to the body; wherein the circuitry is furtherconfigured to detect the electrical energy transferred through the bodyby these electrodes to determine whether the acoustic energy is focusedon the acoustic receiver. The circuitry could have one or more pairs ofelectrodes that are configured to determine whether the acoustic energyis focused on the acoustic receiver. The circuitry could also beconfigured for sequentially transmitting the acoustic energy.

Another exemplary embodiment of the invention described herein is asystem for stimulating tissue comprising an implantable acousticcontroller-transmitter comprising an array of acoustic transducersconfigured to transmit focused acoustic energy; one or more implantableacoustic receiver-stimulators adapted to receive the acoustic energy andconvert the acoustic energy into electrical energy, wherein thereceiver-stimulator further comprises electrodes configured to be inelectrical communication with the tissue; and the electrical energy isdelivered between the electrodes; and wherein the controller-transmitteris configured to determine the location of one or more of thereceiver-stimulators relative to the controller-transmitter so that thecontroller-transmitter can direct the focused acoustic energy to one ormore of the receiver-stimulators.

Another embodiment of this invention is a method and system fordetermining the location of an acoustic receiver in the body. An arrayof acoustic transducers is used to transmit acoustic energy at aspecific location in the body. The acoustic receiver is configured withelectrodes that generate an electric location signal whenever itreceives acoustic energy. Separate detection electrodes can detect theelectric location signal indicating when the array of acoustictransducers is focused on the acoustic receiver and revealing thelocation of the receiver. The transducer array could be configured tosequentially steer the acoustic energy until the location signal isdetected or a preset time limit has been reached. The transmittedacoustic energy could be a focused acoustic beam. The location signalcould be detected by a sensing circuit on the controller-transmitter.

In another embodiment of the invention, the controller-transmitter wouldbe further configured to adjust the transducer array to transmit focusedacoustic energy to the region of the tissue associated with detectingthe location signal. This focused energy could be adequate to stimulatetissue and, in particular, cardiac tissue. In yet another embodiment,this focused energy would be generated based on characteristics of thelocation signal.

In yet another embodiment of this invention, an implantable acousticcontroller-transmitter comprises an adjustable transducer arrayconfigured to transmit acoustic energy into tissue; an implantableacoustic receiver-converter comprises a transducer assembly adapted toreceive the acoustic energy and convert the acoustic energy toelectrical energy, where the transmitter is configured to transmit anacoustic locator signal towards the receiver, and the receiver isconfigured to generate a location signal. The location signal could beeither an electrical output or an acoustic transmission in response tothe locator signal. The locator signal could be focused acoustic energy.Alternatively, the focused acoustic energy that is transmitted by thetransmitter can be converted to electrical energy by thereceiver-converter and stored in the receiver-converter as electricalenergy to be discharged at the appropriate moment. The electrical energycould also be used to operate various circuitry, such as the controlcircuitry, diagnostic sensing circuitry or communication circuitry.

Another exemplary embodiment of this invention is a system forstimulating tissue comprising an implantable acousticcontroller-transmitter with an acoustic transducer array adapted totransmit acoustic energy into tissue; and an implantable acousticreceiver-stimulator which receives acoustic energy and converts theacoustic energy to electrical energy and which has a first electrodeassembly connected to the receiver-stimulator and adapted to be inelectrical communication with the tissue, wherein thereceiver-stimulator periodically transmits a location signal, andwherein the controller-transmitter detects the location signal. Thelocation signal could be an electrical output or an acoustictransmission that could be sensed by the controller-transmitter. Basedon the characteristics of the location signal, the transducer arraycould be adjusted to transmit focused acoustic energy towards thereceiver-stimulator. The characteristics of the location signal wouldinclude frequency, duration, amplitude, phase, and time of flight of thelocation signal. The invention is also a method for optimizing acousticenergy transmission in tissue between an implantablecontroller-transmitter and one or more implantable receiver-stimulatorscomprising transmitting an acoustic locator signal from thecontroller-transmitter towards the receiver-stimulator, wherein thecontroller-transmitter comprises an adjustable transducer array; andgenerating a location signal from the receiver-stimulator in response toreceiving the locator signal. The method could include detecting thelocation signal using the controller transmitter and adjusting thetransducer array. The transducer array can transmit focused acousticenergy towards the receiver-stimulator. Additionally, the method couldinclude adjusting the transducer array sequentially to transmit focusedlocator signals to regions of the tissue until the receiver-stimulatorlocation signal is detected by the controller-transmitter or a pre-settime limit has been reached; and adjusting the transducer array totransmit focused acoustic energy to the region associated with thedetected location signal. The method could further include convertingthe acoustic energy using the receiver-stimulator, and applying theconverted energy to the tissue. The energy could be of sufficientmagnitude to stimulate tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a tissue stimulation system.

FIGS. 2A-B illustrate one embodiment of this invention.

FIG. 3 illustrates the acoustic array scanning a region for locationsignals in response to locator signals.

FIG. 4 shows the phases resolved into different components.

FIGS. 5A-5C show various electrode configurations.

FIGS. 6A-6D show methods for minimizing scan time for target detection.

FIGS. 7A-7C illustrate an embodiment using frequency shifting foracoustic beam steering and optimizing energy transmission.

DETAILED DESCRIPTION

A leadless tissue stimulation system is shown in FIG. 1 as system 100.An implantable or external controller-transmitter module 110 generatesacoustic waves 120 of sufficient amplitude and frequency and for aduration and period such that the receiver-stimulator module 150electrically stimulates tissue. An external programmer 170 wirelesslycommunicates with an implantable controller-transmitter module 110,typically by radio frequency telemetry means 116, to adjust operatingparameters. The implantable controller-transmitter module comprises atelemetry receiver 115 for adjusting the transmit acousticcharacteristics, control circuitry 140 and signal generator 117, a poweramplifier 118, and an output transducer assembly 119 for generating theacoustic beam 120 transmitted to receiver-stimulator 150.Understandably, the controller-transmitter 110 transfers acoustic energyto the receiver-stimulator 150 leadlessly. Control circuitry 140contains an electrical signal sensing circuit element 141 connected toone or more sensing electrodes 145 disposed on the outer casing of thecontroller-transmitter or connected via cables to thecontroller-transmitter. Alternatively, electrical sensing circuit 141may be a typical electrogram sensing circuit or may be an electricalspike detection circuit.

The receiver-stimulator 150 comprises a piezoelectric receivingtransducer 151, rectifier circuitry 153, and tissue contactingelectrodes 155. In this embodiment, acoustic energy received andrectified by the receiver-stimulator is directly applied to theelectrodes 155. Alternatively, the receiver-stimulator module maycomprise multiple transducer/rectifier channels in a variety ofcombinations, which may be in series or parallel orientations, or theconstruction may perform impedance matching, and/or for signal filteringas previously disclosed in co-pending application Ser. No. 11/315,524,to increase the efficiency of the receiver-stimulator.

One embodiment of the present invention is shown in FIG. 2A as system200. The controller-transmitter module 210 is placed either inside thebody, but remote from myocardial tissue, or outside the body in contactwith the body surface. The external programmer 170 communicates with thecontroller-transmitter module, typically by radio frequency telemetry116. The telemetry module 115 inside the controller-transmitter unit 210provides two-way communications directly with the control circuitry 220.A separate continuous wave (CW) signal generator 217 inside thecontroller-transmitter 210 provides the acoustic operating frequency forthe system. The control circuitry 220 and signal generator 217 are bothconnected to each channel of a two dimensional acoustic transducer array260 (shown in FIG. 2B), where each channel comprises a transmit/receivetransducer element 230 ij, a power amplifier 218 ij and phase shiftermodule 240 ij. The phase shifter module 240 ij, ensures that duringacoustic transmissions, each channel transmits with the correct phase soas to form a efficient, focused narrow acoustic beam intended toprecisely intercept the receiver-stimulator. A control signal fromcontrol circuitry 220 defines the transmit phases. The output of thephase shifter 240 ij then passes to the power amplifier 218 ij of thechannel, which is also under the control of the control circuitry 220,and which can be either in an OFF state, a full ON state, or at selectedlevels of intermediate power which might be required for beam shading.The output of the power amplifier passes directly to the channeltransducer element 230 ij. One embodiment of using the phase shifter foreach output channel has been described above. Other techniques can alsobe employed, such as direct formatting of the transmit beam by thecontrol circuitry 220.

The controller-transmitter 210 would scan a spatial region by sendingnarrow acoustic beams (the locator signals), looking for a response (thelocation signal), from the receiver-stimulator. If the focused, directedacoustic beam intersects the receiver-transmitter the acoustic energy isconverted by the receiver-stimulator and delivered as an electricaloutput onto the electrodes 155. This electrical output would generate anelectrical signal that would be detected by sensing electrodes 145 anddetection circuits 241 of the controller-transmitter 210. If thecontroller-transmitter does not detect an electrical signal within areasonable time frame, the inference would be that the directed acousticbeam did not intersect the receiver-stimulator and the directed acousticbeam was “off target.” Such time frames may be predetermined ordetermined based on location signal characteristics. Then, thecontroller-transmitter would adjust the focused, directed beam toanother portion of the region where the receiver-stimulator may belocated, possibly chosen to be close to the previous region, and repeatthe locator signal transmission thereby scanning the spatial regioniteratively. In this manner, an electrical signal will be generated anddetected if the receiver-stimulator is in the spatial region beingscanned. The controller-transmitter then uses the focused, directed beamparameters that resulted in a detected electrical signal (locationsignal) as the target (transmission region) for the efficienttransmission of a narrow acoustic beam of acoustic energy towards thereceiver-stimulator. Alternatively, the controller-transmitter couldthen analyze characteristics of the detected electrical signal todetermine whether the directed transmitter beam was adequately targetingthe receiver-stimulator.

The scanning process is shown in more detail in FIG. 3. The phased array260 of the controller-transmitter is composed of individual transducers230 ij. For convenience the array is oriented in the x-y plane at z=0.The spatial volume to be scanned 305 encompasses all of the possiblelocations for the receiver-stimulator, and, again for convenience, islocated in the z>0 half space with respect to the phased array 260. Theextent of 305 is constrained by anatomical limits and may vary dependingon the specific stimulation application. The spatial volume 305 isbroken up into multiple volumes 302 kl, which are individually scannedor tested. The volumes 302 kl may overlap; however, it is desirable tohave the entire collection of volumes cover the region 305. The array isaimed at a volume 302 kl by setting the appropriate phase parameters forthe array elements 230 ij.

The following method, provided as an example, can be used fordetermining the correct phase parameter for each of the array elements.A spatial location v1 for the volume 302 kl is picked; it is typically,but not necessarily, the center of the volume. A spatial location v2 forthe array element 230 ij is chosen; typically, but not necessarily, thecenter of the array element. Note in general that v1 and v2 are 3Dvectors with x, y and z components. The phase is given by

$\begin{matrix}{\phi = {\left( {2\pi\frac{{{{v\; 1} - {v\; 2}}}_{2}}{\lambda}} \right)\;{mod}\mspace{11mu} 2\pi}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where ∥ ∥₂i is the standard Euclidean norm or distance function and modis the modulo arithmetic operator and λ is the wavelength of theacoustic wave. Alternatively, the phase parameters may not be computedmodulo 2π but rather modulo n2π where n is the maximum phase delay, inwavelengths, across elements of the array 260 when aiming at thefarthest angular extent of region 305. This is slightly more efficientand therefore preferred because the first cycle of the transmitted arraywill be targeted at the volume 302 kl whereas modulo 2π phase results inthe first n cycles of the transmitted wave being out of focus.

Typically the x, y width of each volume 302 kl will be selected as thewidth of the narrowest acoustic beam that is possible from the array260. This minimal acoustic beam width w is approximated by

$w = {\frac{\lambda}{D}r}$where λ is the wavelength of the acoustic wave, D is the lateral size ofthe array 260, and r is the range or distance along the z axis from thearray 260 to the volume 302 kl. Therefore, if the array 260 isrectangular, i.e., different lateral widths in the x and y dimension,then the minimal beam width and hence x and y dimensions of the volume302 kl will be different. Also note that since the minimal acoustic beamwidth increases with range r, the volume 302 kl is in general wedgeshaped, expanding in lateral dimension with increased range r. Theacoustic beam itself tapers off from a center peak rather than endingabruptly therefore it is desirable for the volumes 302 kl to have someoverlap, for example 50% overlap. This provides finer targeting of thereceiver-stimulator and hence more efficient transfer of acousticenergy.

The maximum lateral width, W, of the interrogation region 305 isapproximated by

$W = {\frac{\lambda}{d}r}$where λ is the wavelength of the acoustic wave, d is the lateral size ofan individual array element 230 i, and r is the range or distance alongthe z axis from the array 260 to the volume 302 kl. Similar to theindividual volumes 302 kl the entire scan region 305 has a wedge shapeexpanding out in lateral dimension with increasing range r.

If 305 lies entirely in the far field of the array 260 then depth or zfocusing is not required and each volume 302 kl can be extended over theentire z depth of region 305. However, if 305 overlaps with the nearfield transmission region of the phased array 260, multiple layers ofvolumes 302 kl, 303 kl, etc. must be scanned in the z dimension as well.Generally speaking, the boundary between the near and far field regionsis given by

$r = \frac{D^{2}}{4\lambda}$

Of course, in situations where the possible target location region iseither in the far field or moves only within a fixed focal zone, thenscanning in the z dimension may not be required.

Another method for quickly and efficiently determining the requiredphasing for the elements of the transmit array in thecontroller-transmitter is described below. As described previously, therequired phasing can be calculated; however, this is computationallyexpensive, which consumes valuable energy and time, particularly becauseit involves the calculation of a square root. One alternative is topre-compute the required phases for each element 230 ij of the array 260for each scan location 302 kl. This, however, quickly results in asignificant amount of required memory. There is the additional burden ofthe time required to read the phases out of memory and load them intothe phase shifter 240 ij for each of the array elements 230 ij. Thistime can be reduced by increasing the clock speed of the digitalelectronics in the controller-transmitter or paralleling the loadingprocess.

FIG. 4 describes how the required phases can be broken down into threeseparate components. The first two are phase gradients in the x and ydirection. These are linear functions of the x and y location of thearray elements and hence are relatively inexpensive to compute.

If the receiver-stimulator is very far away from thecontroller-transmitter only these first two components of the phase arerequired. However, if the receiver-stimulator is around the borderregion of the far-field of the array and certainly if it is within thenear field, a third component shown as the pre-phasing component isrequired. This pre-phasing component is not a linear function of theposition of the transmit element within the array and is therefore moreexpensive to compute.

The basic scheme is to calculate the pre-phasing component infrequentlyand to compute the linear component of the phases whenever the arrayneeds to be steered to a new location. Several options exist fordetermining the pre-phasing component. One is to calculate thepre-phasing as the phase required to steer to a centered target(directly perpendicular, no off angle-steering) at a nominal expectedrange (distance) between the controller-transmitter and thereceiver-stimulator. This can be done using the equation (Equation 1)shown above. The pre-phasing compensates for the fact that thereceiver-stimulator is not strictly in the far-field, which is only trueif it is infinitely far away from the controller-transmitter. If it werein the far-field the pre-phasing component would simply be zero, i.e.,all elements in the array transmitting with the same phase. Thesepre-phases can be calculated and stored in read-only memory (ROM) anddownloaded as part of the manufacturing of the controller-transmitter oralternatively determined once when the controller-transmitter isimplanted. The latter scheme has the advantage of more exact knowledgeof the range between the controller-transmitter and receiver-stimulator.

The linear phase gradients can be computed by the control circuitry andthen downloaded to each of the phase controllers 241 ij or the phasecontrollers can determine the linear phase components using either alook up table or dedicated computation circuitry.

Another alternative is to calculate the pre-phasing based on the nominallocation of the receiver-stimulator (i.e., not just the range but alsothe angular location). This works well if the receiver-stimulator islocated at a significant angle from perpendicular to thecontroller-transmitter. If there is not significant movement of thereceive-stimulator relative to the controller-transmitter, thepre-phasing component only needs to be computed once saving significantcomputational overhead.

The electrical output produced through electrodes 155 as part of thescanning process may be considered a stimulation or pacing output, ifsufficient energy is contained in the output to excite the tissueadjacent to electrodes 155; however, it is not required that the tissuebe stimulated to detect the electrical signal at electrodes 145. Infact, it is advantageous for the electrical output to not be astimulating pulse because the energy required to produce an electricaloutput that is detectable by electrodes 145 and detection circuits 241is significantly lower than the energy required to stimulate tissue.This lower energy requirement is primarily achieved by shortening theduration of the locator signal and resulting electrical output atelectrodes 155 to a value that is significantly below that used tostimulate tissue. For example, signal durations for cardiac tissuestimulation are in the range of 200 μs to 2000 μs, while typicaldurations are in the 400 μs to 500 μs range. The minimal duration of alocator signal is affected by various parameters: the operatingfrequency of the system, the Q of both transmitter and receivertransducers as well as the size of the transmit array and overallreceiver structure if it contains multiple transducers. A minimal timeof 10 cycles is a reasonable estimate. For an ultrasound systemoperating in the 500 kHz to 1 MHz frequency range this sets the minimumlocator signal duration at 10 to 20 μs—at least 20 times shorter thanthe typical duration for tissue stimulation. This results in at least 20times less energy used for transmitting the locator signal than thatused to stimulate the tissue, making this embodiment attractive.

Short duration locator signals require different detection circuits 241than that used for conventional ECG processing or even pacing spikedetection. ECG signals are typically processed with an amplifierbandwidth of 0.5 Hz to 100 Hz. Pacing spike detectors typically have abandwidth of 1 kHz to 2.5 kHz. A 10-20 μs electrical signal produced inresponse to 10-20 μs locator signal requires a bandwidth of up to 100kHz.

Research on both animal models and humans indicate that it is common toobserve signal attenuation of 65-80 dB for a pacing signal generatedfrom within the heart and sensed on surface ECG electrodes. Therefore a1 volt electrical pulse delivered across electrodes 155 would result ina 560 microvolt to 100 microvolt signal on electrodes 145. State of theart amplifiers can achieve noise figures in the range of 20nV/(Hertz)^(1/2), resulting in noise on the order of 6 microvolts over a100 kHz bandwidth, resulting in a very reasonable signal to noise ratiofor detection of a location signal. However, such high bandwidth, highgain amplifiers consume more power than conventional ECG amplifierswhich are amplifying lower bandwidth higher amplitude signals. It istherefore advantageous to only turn on these amplifiers when they arerequired, i.e., immediately following transmission of acoustic locatorsignals.

Additionally it is important to note that the location signal isgenerated and sensed from two electrodes that are spatially close toeach other. The positions of both, receiver-stimulator electrodes 155and controller-transmitter electrodes 145, are constrained by practicallimitations. Hence, the electrical signal produced by electrodes 155will have a dipole radiation pattern and the sensitivity of theelectrodes 145 will have a dipole pattern as well. FIG. 5a shows atypical dipole arrangement. Sensing electrodes 145 are input into adifferential amplifier 410. The dashed line 401 indicates a region, a“blind spot”, where a signal source cannot be sensed by the electrodes145. This is because a signal source placed along this line isequidistant to both electrodes 145 and the differential amplifiersubtracts these two equal signals producing a null output.Correspondingly, the dashed line 402 indicates a region where a signalsource will be sensed with maximum output from the amplifier 410. Asimilar behavior occurs as a result of the transmission of theelectrical signal through electrodes 155. Therefore, the overallattenuation will be the result of the superposition of two dipolepatterns. In order to avoid potential “blind spots” in these dipolepatterns it is, therefore, advantageous to use more than two electrodes145 on the controller-transmitter. FIG. 5b shows how the addition of athird electrode eliminates the problem of this “blind spot”. Twoamplifiers 420 a and 420 b are used to amplify signals from two separatedipoles oriented 90 degrees apart. The two signal outputs, 242 a and 242b of 420 a and 420 b, respectively, are then analyzed for the presenceof the location signal. Even more improvements can be made by theaddition of more electrodes that are spatially separated from the firstthree electrodes as shown in FIG. 5c . This has the additional benefitof avoiding any “blind spots” in the dipole pattern generated byelectrodes 155 at the receiver-stimulator. One electrode 146 is chosenas a reference and all other electrodes 145 are amplified relative tothis reference using amplifiers 410 i each of which produces a signal420 i. The dipole signal 244 from any pair of electrodes can then becalculated by taking the difference between two of the output signals420 i, using 243 which can be implemented either as a hardwaredifferential amplifier or in software as the subtraction of twodigitized signals. As discussed above, the amplifiers 410 i arenecessarily high bandwidth, high gain amplifiers and therefore consumesignificant power. Therefore, it is advantageous to only use those thatprovide the largest amplitude location signals. Assuming that the motionof the receiver-stimulator and controller-transmitter will significantlychange the amplitude of the location signal, once the electrode pairthat produces the largest location signal is determined, only theamplifiers used to produce this signal need to be used, significantlyreducing power consumption.

An important consideration is the time taken to determine the locationof the receiver-stimulator. Obviously, this time should be as short aspossible. If this time is comparable to the cardiac cycle, then motionof the heart between determination of the location and subsequentdelivery of stimulation energy becomes problematic. It is alsoadvantageous to minimize the required scan time when the leadlessstimulator is used concomitantly with a standard pacemaker to achievetherapeutic bi-ventricular pacing. In this case, as disclosed in pendingapplication Ser. No. 11/315,023, the controller-transmitter transmitsacoustic energy to stimulate the heart immediately following thedetection of a right ventricular (RV) pacing artifact in theconcomitantly implanted device. Preferably, the determination of thereceiver-stimulator position is done after the detection of the RVpacing artifact so that the effect of cardiac motion between positiondetermination and stimulation is minimized.

FIG. 6 shows several methods for minimizing the required scan time. FIG.6a shows a partition of the space to be scanned into different targetregions. The partition assumes there is no depth targeting and thereforethe scan space is in the x-y plane at a fixed z location. The method canbe easily extended to the case of depth targeting. The speed of sound inthe soft tissue and blood is approximately 1.5 mm/μsec. Considering alarge distance between the controller-transmitter andreceiver-stimulator of 200 mm results in a maximum time of flight ofapproximately 133 μsec. FIG. 6b shows a simple scan method where thetime between locator signals, P, is chosen to be greater than theexpected time of flight. A method for processing the output signal 244is described as follows. Detection of the location signal 510, followinga locator signal 505, indicates that the receiver-stimulator iscontained in the volume corresponding to the locator signal 505 (volume15 in FIG. 6a ). Furthermore, the time of flight which is proportionalto the range between the controller-transmitter and thereceiver-stimulator can be measured by the time delay betweentransmission of the locator signal and detection of the location signal.

FIG. 6c demonstrates a faster scanning method. In this case, the timebetween transmit pulses (locator signals), P, is shorter than the actualtime of flight and is limited only by the duration of each individuallocator signal and the setup time for the controller-transmitter toprepare for the next locator signal. This results in multiple locatorsignals in flight simultaneously between the controller-transmitter andreceiver-stimulator, considerably reducing the scanning time. Once alocation signal is detected, determination of the actual locator signalthat produced the location signal requires knowledge of the nominal timeof flight between the controller-transmitter and receiver-stimulator asshown in FIG. 6c . This optimized scheme is applicable if the previouslocation and hence time of flight to the receiver-stimulator is knownand only small movements of the receiver-stimulator relative to thecontroller-transmitter are expected. The time between locator signals,P, can be set to the maximum expected range of motion. For example, ifthe maximum possible motion is 40 mm then P must be at least 40/1.5 or27 μsec.

During initial operation, when the location of the receiver-stimulatorand hence nominal time of flight is totally unknown, a hybrid techniqueas shown in FIG. 6d can be used. A rapid scan of the entire region isperformed using a technique similar to that shown in FIG. 6c until alocation signal is detected. Once a location signal is detected a slowerscan similar to that shown in FIG. 6b is performed for the volumes nearthe detected location signal (starting with volume 13 then 14, etc.)This will pinpoint the exact volume (in this case 15) and allow backcalculation of the actual time of flight.

In some cases, a longer duration between locator signals than that usedin FIG. 6b may be required. This can happen if there is sufficientacoustic energy from the locator signal is reflected off anatomicalstructures in the body and the receiver-stimulator responds to thesereflected locator signals. Generally, this is handled by increasing thetime between locator signals such that any reverberation or reflectionfrom a previous locator signal has decayed before transmitting anotherlocator signal. However, the likelihood of this problem occurring can besubstantially reduced by prior knowledge of the nominal time of flight.This allows the controller-transmitter to look for a location signalover a narrow time window eliminating false detections due to reflectedlocator signals.

Another strategy for minimizing the scan time and the energy expended onthe scan itself is to perform an intelligent search. One approach is tostart the scan by transmitting a locator signal to the previous knownposition of the receiver-stimulator. Therefore, if thereceiver-stimulator has not moved outside of the scan volume, only onelocator signal is required. If more scanning is required, anotherstrategy is to expand the search out from the last known position forthe receiver-stimulator. Another approach is to remember the previoushistory of motion of the receiver-stimulator and use this tointelligently scan for it. This will greatly reduce the number of scanswhenever the primary motion of the receiver-stimulator is periodic forexample due primarily to cardiac and respiratory motion.

It should be noted that more than one receiver-stimulator could beimplanted and operated using the different approaches described abovefor optimizing energy transmission. The location of eachreceiver-stimulator relative to other receiver-stimulators can beregistered during the time of implantation. Following implantation, whenthe receiver-stimulators move due to cardiac motion, breathing, etc.,they are likely to move in concert with each other. However, therelative location of the receiver-stimulators with respect to thecontroller-transmitter, which impacts the optimal energy transmission bythe controller-transmitter, is likely to change due to cardiac motion,breathing, etc. To address this issue, if the location of the firstreceiver-stimulator is identified using one of the approaches describedabove, the location of the other receiver-stimulators is immediatelycomputed, based on the relative location of the otherreceiver-stimulators that was registered during implantation.

Alternatively, each receiver-stimulator (when multiplereceiver-stimulators are implanted) can be “addressed” using a locatorsignal with a unique frequency or phase. The approaches describedearlier can then be used sequentially for each receiver-stimulator tooptimize the energy transmission from the controller-transmitter. Ormore simply, if multiple receiver-stimulators are implanted withsufficient difference in location, each could be located directly by thepreviously described methods, based on knowledge of previous locationand the fact that relative locations between devices are unlikely tochange significantly.

While the location signal has been detailed as an electrical signal itshould be understood that the location signal may be of any nature thatcan be detected by a controller-transmitter. For example it could be apassive echo from the device or the receiver could be adapted totransmit an acoustic signal in response to the locator signal.

Another embodiment of the invention described here for optimizing energytransmission from a controller-transmitter is illustrated in FIGS.7A-7C. In FIG. 7A, one element of the controller-transmitter array (“C-TArray”) transmits a wide beam acoustic burst (locator signal), which isreceived by the receiver-stimulator (“R-S”). As the signal is beingreceived in the receiver-stimulator, it is frequency shifted andretransmitted isotropically back to the controller-transmitter, asdepicted in FIG. 7B. This retransmission occurs as the locator signal isbeing received by the receiver-stimulator. The location of thereceiver-stimulator with respect to each element of the C-T array isrecorded into the memory of the controller-transmitter as the detectedphase received per channel. Then, at the clinically appropriate time,the controller-transmitter uses the recorded phase measurements totransmit acoustic energy as a focused beam to the receiver-stimulator tobe delivered to electrodes for stimulation of tissue, as shown in FIG.7C.

The amount of energy contained in the locator signal generated from asingle element in the phase measurement mode described above may besubstantially greater than that used for stimulation. However, becausethe correct phase measurements have been obtained, significantly lessenergy will be transmitted for the stimulation by the entire array thanwould have been required to achieve the same level of energy deliveredto the tissue using a wide beam. Now each element of the array wouldtransmit a focused beam that is much more efficient, compared to thewide beam each element would have transmitted in the absence of thecorrect phase measurement. Additionally, in the method described above,phase measurements were obtained without additional computations, thusfurther minimizing the energy consumption.

Upon creation of the focused beam used for stimulation, not all elementsof the array need necessarily be driven at the same amplitude. If onepathway or the other from the receiver-stimulator to the array ofelements shows either more or less attenuation, this may be overcome bytransmitting with either more or less energy, respectively, or bycompletely turning off severely impacted array elements. Further, it iswell known in the art of array design, that aperture shading (loweramplitude emissions from the edges of the array) has the effect offlattening the acoustic beam, for a greater uniformity within the beam.This can also be accomplished, guided by pre-programmed computations inthe controller-transmitter.

Additional aspects of the invention are described below. In oneembodiment where no locator signal is required, the receiver-stimulatorfirst receives acoustic energy from the controller-transmitter, storespart of the received energy and directs the rest to the tissue. Thestored energy could be anywhere from 0 to 100%, and ideally about 5%, ofthe received energy. Based on a variable, fixed or periodic timeoutwithin the receiver-stimulator, but before the next transmission ofacoustic energy from the controller-transmitter, the stored energy isused by the receiver-stimulator to generate a location signal. Thelocation signal may be an electrical signal, or it may be an acoustictransponder signal transmitted to the controller-transmitter, or asimilar signal generated by the receiver-stimulator as a homing beaconto signal the location of the receiver-stimulator. Thecontroller-transmitter receives the location signal and computes thelocation of the receiver-stimulator, using information, such asamplitude, phase, arrival time, or the like from the location signal.Having identified the location of the receiver-stimulator, thecontroller-transmitter is then able to focus the transmitted acousticbeam to the location or region of the receiver-stimulator and therebytransmit energy or exchange communication optimally.

Alternatively, the controller-transmitter transmits a locator signal inthe form of sufficient acoustic energy to a passive receiver-stimulatorthat uses all the energy received to generate a location signal. In thisembodiment the receiver-stimulator would be adapted to have a statemachine that switches between using acoustic energy for location signalsand using acoustic energy for functional purposes such as stimulation.The location signal is received by the controller-transmitter, whichdetermines the location of the receiver-stimulator based on signalcharacteristics contained in the location signal and then generates afocused beam that is targeted at the location or region of thereceiver-stimulator.

As indicated above, it should be noted that the acoustic receiver of thepresent invention can function as a receiver-stimulator or areceiver-converter, where the receiver-converter can act as a diagnostictool. While the examples illustrate the receiver-stimulator embodiments,the energy optimization techniques described above are equallyapplicable for a receiver-converter.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention, which is defined by the appendedclaims.

What is claimed is:
 1. A method of locating an acousticreceiver-stimulator, the method comprising: transmitting, via acontroller-transmitter, focused acoustic energy to a tissue location;receiving, at the receiver-stimulator, the focused acoustic energy;converting, via the receiver-stimulator, the focused acoustic energyinto electrical energy; delivering, via the receiver-stimulator, theelectrical energy to tissue; and detecting, via one or more electrodesof the controller-transmitter that are electrically connected to thetissue, the delivered electrical energy to determine whether thereceiver-stimulator is present at the tissue location.
 2. The method ofclaim 1 wherein the method further comprises, after determining that thereceiver-stimulator is present at the tissue location, adjusting thecontroller-transmitter to transmit additional focused acoustic energy tothe tissue location.
 3. The method of claim 2 wherein adjusting thecontroller-transmitter includes adjusting an array of acoustictransducers to transmit the additional focused acoustic energy to thetissue location.
 4. The method of claim 1 wherein the method furthercomprises, after determining that the receiver-stimulator is present atthe tissue location, transmitting additional acoustic energy to thetissue location, via the controller-transmitter, to stimulate thetissue.
 5. The method of claim 1 wherein the tissue location is one of aplurality of tissue locations, and wherein transmitting the focusedacoustic energy includes sequentially steering the focused acousticenergy to individual ones of the tissue locations until the deliveredelectrical energy is detected.
 6. A method of locating an acousticreceiver-stimulator, the method comprising: transmitting, via acontroller-transmitter, focused acoustic energy to a tissue location;receiving, at the receiver-stimulator, the focused acoustic energy;converting, via the receiver-stimulator, the focused acoustic energyinto electrical energy; delivering, via the receiver-stimulator, theelectrical energy to tissue; and detecting, via a spike detector of thecontroller-transmitter, an electrical spike indicative of the deliveredelectrical energy to determine whether the receiver-stimulator ispresent at the tissue location.
 7. A method for transmitting acousticenergy into a body, the method comprising: transmitting acoustic energyinto a region of the body; receiving the acoustic energy at an acousticreceiver located in the region of the body; converting the acousticenergy into electrical energy; delivering the electrical energy into theregion of the body; and detecting the delivered electrical energy viaone or more electrodes of an acoustic transmitter that are in contactwith the body to determine whether the transmitted acoustic energy isfocused on the acoustic receiver.
 8. The method of claim 7 whereintransmitting the acoustic energy includes transmitting the acousticenergy via the acoustic transmitter; converting the acoustic energy intoelectrical energy includes converting the acoustic energy intoelectrical energy via circuitry of the acoustic receiver; and deliveringthe electrical energy into the region of the body includes deliveringthe electrical energy via at least one electrode of the acousticreceiver that is electrically connected to the region of the body. 9.The method of claim 7 wherein the method further comprises, afterdetermining that the acoustic energy is focused on the acousticreceiver, transmitting additional acoustic energy to the acousticreceiver.
 10. The method of claim 9 wherein the method furthercomprises: converting the additional acoustic energy into additionalelectrical energy; and delivering the additional electrical energy intothe region of the body to stimulate tissue at the region in the body.11. The method of claim 10 wherein the tissue is cardiac tissue.
 12. Themethod of claim 7 wherein the method further comprises implanting theacoustic receiver in the region of the body.
 13. The method of claim 7wherein detecting the delivered electrical energy via the one or moreelectrodes further includes detecting an electrical spike via a spikedetector of the controller-transmitter electrically coupled to the oneor more electrodes.
 14. A method for transmitting acoustic energy intotissue, the method comprising: transmitting an acoustic locator signalinto the tissue; receiving the acoustic locator signal at an acousticreceiver implanted in the tissue; generating an electrical locationsignal; transmitting the electrical location signal into the tissue;detecting the electrical location signal via one or more electrodes ofan acoustic transmitter that are electrically connected to the tissue;and transmitting focused acoustic energy toward a region of the tissueassociated with the detected electrical location signal.
 15. The methodof claim 14 wherein transmitting the focused acoustic energy toward theregion of the tissue includes adjusting a transducer array to transmitthe focused acoustic energy based on a characteristic of the electricallocation signal.
 16. The method of claim 14 wherein transmitting theacoustic locator signal includes transmitting a wide beam acousticburst.
 17. The method of claim 14 wherein the tissue is cardiac tissue.18. The method of claim 17 wherein the method further comprises:converting the focused acoustic energy into electrical energy; anddelivering the electrical energy into the region of cardiac tissue tostimulate the cardiac tissue.
 19. A method for transmitting acousticenergy into tissue, the method comprising: transmitting an acousticlocator signal into the tissue; receiving the acoustic locator signal atan acoustic receiver implanted in the tissue; generating an electricallocation signal; transmitting the electrical location signal into thetissue; detecting the electrical location signal; and transmittingfocused acoustic energy toward a region of the tissue associated withthe detected electrical location signal, wherein transmitting thefocused acoustic energy toward the region of the tissue includesadjusting a transducer array to transmit the focused acoustic energybased on a characteristic of the electrical location signal, and whereinthe characteristic is at least one of a frequency, duration, amplitude,phase, and time of flight of the electrical location signal.
 20. Amethod for transmitting acoustic energy into tissue, the methodcomprising: transmitting an acoustic locator signal into the tissue;receiving the acoustic locator signal at an acoustic receiver implantedin the tissue; generating an electrical location signal; transmittingthe electrical location signal into the tissue; detecting the electricallocation signal by detecting an electrical spike at one or moreelectrodes electrically connected to the tissue; and transmittingfocused acoustic energy toward a region of the tissue associated withthe detected electrical location signal.